1. Field of Invention
The invention relates to a power module, and more particularly to a power module applied to a power converter.
2. Related Art
There are more kinds of power converters due to different applications of the power converters. The power converters may be classified into a non-isolated AC/DC power converter, a non-isolated DC/DC power converter, an isolated DC/DC converter, an isolated AC/DC power converter and DC/AC, AC/AC power converters according to the type of the electric energy being converted. The non-isolated AC/DC power converter is, for example, composed of an AC/DC conversion circuit for a power factor correction (hereinafter referred to as PFC) circuit. The isolated AC/DC power converter is composed of one PFC circuit and one or multiple DC/DC converters. Because the electric energy properties to be converted and the conversion levels are different, the power densities and efficiencies, which can be easily achieved by various converters, are not always the same. Taking the isolated AC/DC power converter as an example, the general industrial power density is 10 W/inch3, and the efficiency is about 90%. The non-isolated AC/DC power converter, the isolated DC/DC converter and the DC/AC power converter have the higher efficiencies and power densities.
The high efficiency of the power converter represents the low energy consumption. If the efficiency is 90%, the converted energy consumption is equal to about 10% of the total input energy of the power converter. If the power converter has the efficiency of 91%, its converted energy consumption is reduced to 9% of the total input energy. That is, when the efficiency is increased by one point, the energy consumption is reduced by 10% as compared with the power converter with the efficiency of 90%, and the improvement is very considerable. In fact, the efforts on the efficiency improvement of the power converter are often proceeded with the order of magnitude equal to 0.5% or even 0.1%.
The energy consumption of the power converter is mainly composed of the on-state loss and the switch loss, especially the switch loss of the active device. The switch loss is more significantly affected by the working frequency. The power converter, especially the switch power converter, has the working frequency usually higher than 20 kHz in order to decrease the audio noise. The selection of the actual working frequency of the power converter is more significantly affected by the inactive device, especially the magnetic element. If the magnetic element has the small size, the high frequency is usually needed to decrease the magnetic flux density of the working element in order to achieve the reliable work. Thus, the high switch loss is induced. Alternatively, the wire diameter of the wire set and the number of loops in the magnetic element can be increased so that the on-state loss is increased and the high loss is further induced. On the contrary, if the magnetic element has the large size, the working frequency can be lowered under the precondition of assuring the reliable work, and the switch loss is thus decreased. Also, the wire diameter of the wire set and the number of loops in the magnetic element may be decreased, so that the on-state loss is decreased, the total loss is decreased and the high efficiency is obtained.
Therefore, it is easy to understand that one of the key factors of obtaining the high power density or the high efficiency is to enhance the space availability inside the power converter. As the space availability gets higher, the larger space for the inactive device, especially the magnetic element, which is very important to the power converting efficiency, is left. Thus, the large-size inactive element can be easily used so that the power efficiency is increased. Also, the total power of the power source can be increased by using the large-size inactive device, so that the power density of the power converter can be enhanced. Thus, for the high power space availability, the high efficiency can be achieved more easily under the specific power density, or the high power density can be achieved more easily under the specific efficiency, and it is possible to possess both the high power density and the high efficiency concurrently.
The semiconductor device is one of the important factors for determining the efficiency of the power converter. However, the use of the semiconductor device tends to unavoidably need to use the additional materials, such as the package material for protecting the semiconductor, the heat sink for heat dissipating, the fixture for fixing the semiconductor device, and the like, which are not useful to the power converting efficiency. As the ratio of these materials to the power converter gets greater, the internal space availability of the power converter gets worse. At present, the excellent product has well utilized the internal space of the power converter. As a result, the ratio of the space, occupied by the power semiconductor device, to the total size of the power converter gets larger and larger, and gets more and more emphasized.
At present, many advanced techniques in the industry have been disclosed. For example, the heat sink is optimized, and the mounting is simplified to reduce the space occupied by the heat sink and the space for mounting. For example, a new insulation washer technique is provided, and a screw and a fixture are eliminated to reduce the size and improve the design of the power converter.
In order to enhance the power performance, the space availability has to be continuously enhanced. The package space availability of the semiconductor device becomes a bottleneck. For an integrated power module (IPM), many semiconductor devices are integrated within a device package to provide a possibility for the enhancement of the space availability within the package. The integrated modules have different integrated contents because the applications are different. One single power semiconductor device and its controller or driver may be integrated together. Multiple power semiconductor devices may be integrated together. Many semiconductor devices and their corresponding controllers or drivers may be integrated together. The different integrated contents cause different consideration points and difficulty levels. For the sake of distinguishing, the power module mentioned hereinbelow includes two power chips to emphasize the integration of multiple power chips.
The power module with the integrated power device may further be integrated with some controlling and driving devices in some occasions. The frequently used power chips include the MOSFET, IGBT, the power diode and the like. The controlling and driving elements often include a few transistors, ICs, passive devices and like. Because multiple devices are integrated as one device, the power module has the advantages including the convenient usage and the long time without fault, and is widely applied to various occasions. Because the power module has many power chips integrated together, the generated heat is high and the power chips are distributed in many points. The thermal management thereof thus becomes very important. Among many existing arts, the heat dissipating ability is optimized.
In a first existing art, an internal cross-sectional view of a typical power module 30 is as shown in FIG. 1. Element devices 32 and 34 and a lead frame 35 are assembled in the existing art. Taking some chips 32, 34 of the power chip as an example, its front-side electrode can be electrically connected to the lead frame through wire bonding, copper strap bonding, or the like, and its backside can be electrically and/or mechanically connected the pin-frame through strap bonding, silver paste, sintering, epoxy adhesive or the like. After the element device and the lead frame are assembled, the areas to be protected are covered by a molding compound 36 so that the mechanical, dustproof, moisture-proof, insulation protection functions may be achieved. This structure has the advantage of the low price.
In the existing art, the heat dissipating surface insulated by the molding compound plays the role of the mechanical protection at the same time. So, the thickness thereof is thicker, and is usually greater than 0.5 mm. In general, the thermal conductivity of the molding compound is about 1 W/m·K. The thermal conductivity from the chip surface to the case is calculated as follows:
  R  =                    Thermal        ⁢                                  ⁢        conducting        ⁢                                  ⁢        distance                    Thermal        ⁢                                  ⁢        conductivity        ×        area              =                            t          ⁢                                          ⁢                      (            mm            )                                                k            ⁡                          (                                                w                  /                  m                                ·                k                            )                                ×          A          ⁢                                          ⁢                      (                          mm              2                        )                              ×      100      
For the area with the size of 10 mm by 10 mm and the thickness of 0.5 mm, if the thermal conductivity of the molding compound is 1 W/m·K, the thermal resistance can reach as high as 5 K/W. As a result, the package usually has the worse heat dissipating performance. That is, the thermal resistance (Rjc) from the junction of the chip to the case is greater in the example of the power semiconductor device. In addition, because the molding compound has the smaller coefficient of heat conductivity, its transversal thermal diffusing ability is also lower. Thus, the heat concentration spot (hot spot) tends to occur, thereby decreasing the device reliability and lifetime.
Therefore, the first existing art has the poor heat dissipating ability, and is not suitable for the occasion requiring the high heat dissipating ability. In order to optimize the performance of the power module, many techniques have been further proposed.
In a second existing art, as shown in FIG. 2, a heat sink 31 is added to one side of the molding based on the first existing art. Because the heat sink has the higher thermal conductivity (e.g., the thermal conductivity of the copper is higher than 300 W/m·K), the average temperature performance of the module is increased and the problem of the hot spot can be eased by a predetermined level so that the thermal management ability of the module is increased. However, the heat sink is usually requested to be electro-insulated, the molding compound 36 is usually filled in between the wire frame 35 and the heat sink. Due to the limitation of the molding technique, the thickness of the molding compound layer is typically greater than 0.2 mm, usually greater than 0.3 mm. According to the calculating method of the first existing art, the thermal resistance corresponding to the 10 mm×10 mm area is about 3K/W. That is, although the overall heat dissipating performance of this structure is improved, the performance thereof is still poor.
In a third existing art, as shown in FIG. 3, a circuit pattern is formed on a direct bonded copper (DCB) ceramic substrate 31a, which serves as a mounting carrier of the element devices. The element devices 32 and 34 are assembled with the DBC ceramic substrate. For a portion of the semiconductor chip, the wire bonding technique has to be adopted to accomplish the electrical signal connections between the front-side electrodes of the semiconductor chips 32 and 34 and the DBC substrate/lead frame 35. The essence of the structure is based on the second existing art, and adopts the ceramic medium layer with the higher heat conductance coefficient to replace the molding compound layer. Because the frequently used aluminum-oxide ceramic has the coefficient of heat conductivity equal to about 24 W/m·K, which is a great improvement with respect to 1 W/m·K of the molding compound. For the DBC substrate with the 10 mm-by-10 mm area (the ceramic thickness is 0.38 mm, and the copper layers on two sides have the thickness of 0.3 mm), its thermal resistance is 0.17K/W, which is relatively enhanced with respect to 5K/W in the first existing art, and the reduction is higher than 90%.
However, all the element devices 32 and 34 need to be mounted on the DBC substrate, so the required area of the DBC carrier is larger, and the price of the DBC substrate is higher, so that the cost is higher. Because the production technique of the DBC substrate is the high-temperature sintering, the DCB substrate is the product with the high energy consumption, and the use of the large area DBC substrate cannot satisfy the technological progressing trend of the current green environment protection. In addition, the aluminum oxide has the coefficient of heat conductivity equal to about 24 W/m·K, which has been improved as compared with that of the molding compound (usually lower than 1 W/m·K). However, compared with metal (e.g., the copper with the coefficient equal to about 300 W/m·K), the coefficient difference therebetween is still very large so that the transversal heat diffusion ability is not good enough, and the poor thermal uniformity tends to occur. Thus, the heat dissipating performance still can be further enhanced.
In a fourth existing art, as shown in FIG. 4, this structure is improved based on the third existing art, wherein a heat sink 31b is further assembled to the other side of the element device 31a opposite the side where the DBC substrate is assembled. This can enhance the average temperature performance of the module. However, due to the application of the large-area DBC substrate, the warpage, induced by the mismatch between the coefficients of thermal expansion (CTEs) of the DBC substrate, the heat sink 31b and the molding compound 36, may be larger, thereby decreasing the reliability. If the DBC substrate has the too-large dimension, and the DBC substrate and the heat sink 31b are generally soldered together, the defect of too many bubbles in the solder layer may occur. In addition, the problem of the high cost still cannot be solved.
In a fifth existing art, as shown in FIG. 5, a controller or a driver is integrated with the fourth existing art. Because the controller and the driver itself have the energy consumption that is not high, and are more sensitive to the temperature, they are usually designed to be thermally insulated from the material with the higher temperature. In the existing art, the controller 38 or the driver portion serves as one unit (integrated through the PCB substrate or IC), which is connected to the heat sink 31a through a thermally insulated body (the thermally insulated body made of a PCB, a molding material or a dedicated filler, which usually has the coefficient of heat conductivity smaller than 1 W/m·K). The insulation body is formed by way of adhering or filling, or may be coated on the surface. Thus, the device, such as the controller or driver, having the low self power consumption and being more sensitive to the heat, can be reliably used in the package body and be free from the influence of the high temperature of the power chip, so that it can be integrated into the power module and be reliably used.
As mentioned hereinabove, the current power module has the insulating case under the consideration of the commonality so that the mounting and the selection of the heat sink can be simplified. Similar to the fourth existing art, the case is a good electrical conductor (e.g., copper), which is often designed to be electro-insulated. Thus, the metal material, such as copper, in the module tends to provide the single electroconductive function (lead frame, DBC copper layer) or the single heat dissipating function (copper heat sink). The application of the copper layer providing the electro-conductive property and the function of exchanging the heat with the environment directly is rarely seen. Thus, the potential of the material is not completely discovered, so that the space availability is reduced.
In addition, in order to simply the user in mounting the heat sink, screws or fixtures may be adopted to fix the power module to the heat sink. So, the power module is usually designed to withstand the greater mechanical stress. To ensure the reliable usage, the power module is usually designed to have the thicker molding compound to withstand the greater stress. This increases the thickness and the material cost, and greatly reduces the space availability. In addition, the power module is usually requested to have the higher surface smoothness to reduce the stress induced when the heat sink is mounted, thereby increasing the design cost and the mold cost.
Accordingly, it is obtained that the conventional power modules still have various problems including the poor heat dissipating performance, the material wastage, the difficulty of the reliability design, the electrical performance that cannot be optimized, the over design caused by the over-consideration of the commonality, the economic performance that is not high, or the like. More particularly, the space availability is insufficient and the application popularization in the high power density or the high efficiency occasion is thus restricted.
Thus, the performance of the conventional power module still cannot satisfy the requirement of the high power density or the high efficiency power.
For each semiconductor package, the costs to be invested at the beginning are very high, wherein the costs may include, for example, the mold cost, the manufacturing line building cost and the like. So, if the semiconductor package is requested to have the reasonable price, a lot of products have to support and share the investment at the beginning and further to decrease the manufacturing cost. So, the present power module is frequently used in some occasions with the standard applications. For example, the IGBT three-phase bridge module shown in FIG. 6 is widely applied to the occasions of inverters. The circuits in these occasions are standardized and the requirements thereof are uniform, and the required number of productions is very great. So, the semiconductor factory can provide the standardized packages to be selected by the customers.
In the occasion of the power converter, a power module has been successfully used, as shown in the biphase rectifying bridge of FIG. 7. Because most AC/DC power converters need the input rectifying bridges, a lot of power modules are required. In addition, the rectification circuit is quite standardized, and the semiconductor factory can provide the standardized packages to be selected by the customers.
For the power semiconductor devices in other portions of the power converter, many factories try to provide the power modules, but only a few are popularized. In addition to the insufficient performance of the existing art, another critical reason is that the circuit structure of the power converter is complicated and the power converter cannot be easily standardized. If the power modules are provided according to one circuit design, the cost of the power module is higher because only a few power modules are manufactured. Thus, the application there of is restricted.
In order to enhance the power density or converting efficiency of the power converter, a reasonable solution to the power module with the high space availability and the reasonable cost is required. The existing arts, however, cannot easily satisfy these conditions.